Soluble oligomeric forms of amyloid-[unreadable] (A[unreadable]) have been isolated from brain, plasma and CSF and these assemblies appear to correlate with the severity of Alzheimer's disease (AD). To further understand how A[unreadable]3 assembly state influences activity, we have developed structurally defined preparations of oligomeric and fibrillar A[unreadable]31-42. In vitro, these preparations induce distinct neurotoxic, inflammatory, and synaptic plasticity responses. However, the relative contribution of fibrillar and oligomeric A[unreadable] to the pathogenesis of AD remains unresolved. We hypothesize that A[unreadable] exists in at least two states in the brain a diffusible oligomeric state, and an insoluble fibrillar (polymeric) state, and the ratio and clearance of these conformational species is altered in AD patients, promoting neurodegeneration. To dissect this mechanism, conformational-specific antibodies that can distinguish between fibrillar and oligomeric aggregation states of the peptide are critical. We used oligomeric A[unreadable]31-42 as an antigen and an antigen/antibody screen has yielded one oligomer-specific monoclonal antibody (MOAB-1). MOAB-1 does not recognize fibrils by antigen/antibody blotting and ELISA. By Western analysis of SDS-PAGE, MOAB-1 does not recognize A[unreadable] monomer in unaggregated or oligomeric samples, and little immunoreactivity is detected in the fibril samples. Accomplishing the following specific Aims will further test our hypotheses: [unreadable] 1. We propose to isolate and biochemically characterize additional monoclonal antibodies specific for oligomeric or fibrillar A[unreadable]1-42. [unreadable] 2. We will determine the effect of selected antibodies on oligomer- or fibril-induced neurotoxicity, neuroinflammation, and neuroplasticity. We expect that oligomer-specific monoclonal antibodies will inhibit oligomer and not fibril-induced effects and vice versa for the fibril-specific antibodies. [unreadable] 3. We will test the specificity and suitability for immunohistochemistry (IHC) of selected antibodies in rat brains that have been stereotaxically injected with oligomeric or fibrillar A[unreadable]1-42. We expect oligomeric antibodies to immunolabel only the brains injected with A[unreadable] oligomers and not fibril-injected brains, and vice versa. [unreadable] 4. Selected antibodies will be utilized in quantitative IHC and Laser Scanning Confocal Microscopy to determine their localization in hippocampal sections from the brains of neuropsychologically well-characterized patients through the NU-CNADC Neuropathology Core at the Feinberg School of Medicine. [unreadable] 5. An ELISA capture assay will be designed, using existing and novel antibodies, to facilitate measurement of soluble A[unreadable] oligomers in extracts of brain, CSF, and sera. [unreadable] [unreadable] [unreadable] [unreadable]